The invention relates to a method of inspecting pressure vessels for corrosion wastage and other forms of degradation and more particularly to a method of inspecting the regions of pressure vessels adjacent penetration tubes installed with clearance fits and welded to the pressure vessels.
The penetration tubes welded to pressure vessels may become susceptible to stress corrosion cracking after years of on-line operation at high temperatures and high pressures. Micro-cracks may form and grow into leak paths through which the contained fluids may seep and eventually corrode the pressure vessels. Thus, it has been found that the penetration tubes extending through the heads of reactor pressure vessels in the primary systems of pressurized water nuclear reactors are susceptible to cracking. In one case, it was found that a crack had grown in a penetration tube beyond its J-groove weld and that primary water (which is a dilute boric acid solution) had leaked through the crack and corroded the shell of the pressure vessel so that a stainless steel liner was the only structure maintaining the pressure of the system. The nuclear industry now inspects the wetted surfaces of the heads of certain reactor pressure vessels in the course of each refueling outage in accordance with NRC Order EA-03-009. Thus, the wetted surfaces of the heads are inspected visually and the portions of the penetration tubes from the J-groove welds to two inches above the J-groove welds are inspected using ultrasonic, eddy current or dye penetrant techniques.
The Applicants have realized that the inspections now conducted by the nuclear industry may not detect chemical wastage or other degradation in the regions of pressure vessel penetrations adjacent clearances behind some penetration tubes (such as vents in the their heads) until boric acid residues from evaporated leaking water are visually detected on the outer surfaces of the heads during refueling outages.